Braking apparatus for vehicle with collision avoidance mechanism

ABSTRACT

A braking apparatus for a vehicle is provided which includes a hydraulic booster to make wheels of the vehicle produce frictional braking force, a solenoid valve, and a collision avoidance controller. The solenoid valve selectively exerts the hydraulic pressure of brake fluid stored in an accumulator on a spool valve in the booster. When determining that there is a risk of a collision with an obstacle, the collision avoidance controller opens the solenoid valve to achieve emergency braking to minimize the risk of the collision. Basically, emergency braking is achieved by installing the solenoid valve to selectively exert the hydraulic pressure on the spool valve, thus allowing an emergency avoidance braking system to be constructed with a minimum of equipment and facilitating the mountability of the braking apparatus in the vehicles.

CROSS REFERENCE TO RELATED DOCUMENT

The present application claims the benefit of priority of JapanesePatent Application No. 2014-14194 filed on Jan. 29, 2014, the disclosureof which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1 Technical Field

This disclosure relates generally to a braking apparatus for vehicleswhich works to control braking force applied to, for example, anautomobile.

2 Background Art

Japanese Patent First Publication No. 2012-192776 has proposed acollision avoidance system which works to automatically brake a vehicleto avoid an accident with an obstacle present ahead of the vehicle whenit is determined that there is a probability of collision with it.

Japanese Patent First Publication No. 2011-240872 teaches a brake systemfor automobiles which is equipped with a brake simulator and a hydraulicbooster. The brake simulator works to imitate an operation of a typicalbrake system, that is, make the driver of the vehicle experience thesense of depression of a brake pedal. The hydraulic booster serves tocreate a master pressure using the pressure of brake fluid in anaccumulator in response to depression of the brake pedal. The masterpressure is delivered to friction braking devices installed in thevehicle.

The collision avoidance system, as taught in the former publication, isexpected to be used with a typical automotive brake system. The formerpublication is silent about how to employ the collision avoidance systemwith the hydraulic booster. A combination of the collision avoidancesystem and the hydraulic booster, therefore, would result in increasednumber of solenoid valves, hydraulic pipes, or control mechanism makingup the brake system to achieve the collision avoidance, thus leading tolowered mountability of the brake system in automotive vehicles.

SUMMARY

It is therefore an object to provide a braking apparatus with acollision avoidance system which is easy to mount in a vehicle.

According to one aspect of this disclosure, there is provided a brakingapparatus for a vehicle such as an automobile. The braking apparatuscomprises: (a) a master cylinder having a length with a front and arear, the master cylinder including a cylindrical cavity extending in alongitudinal direction of the master cylinder; (b) an accumulator whichconnects with the cylindrical cavity of the master cylinder and in whichhydraulic pressure of brake fluid is stored; (c) a reservoir whichconnects with the cylindrical cavity of the master cylinder and in whichthe brake fluid is stored; (d) a master piston which is disposed in thecylindrical cavity of the master cylinder to be slidable in thelongitudinal direction of the master cylinder, the master piston havinga front oriented toward the front of the master cylinder and a rearoriented to the rear of the master cylinder, the master piston defininga master chamber and a servo chamber within the cylindrical cavity, themaster chamber being formed on a front side of the master piston andstoring therein the brake fluid to be delivered to a friction brakingdevice working to apply a frictional braking force to a wheel of avehicle equipped with this braking apparatus, the servo chamber beingformed on a rear side of the master piston; (e) a spool valve which isdisposed on the rear side of the master piston within the cylindricalcavity of the master cylinder to be slidable in the longitudinaldirection of the master cylinder in response to an effort, as producedby a driver of the vehicle, on a brake pedal so as to switch among apressure-reducing mode, a pressure-increasing mode, and apressure-holding mode, the pressure-reducing mode being to establishfluid communication between the servo chamber and the reservoir, thepressure-increasing mode being to establish fluid communication betweenthe servo chamber and the accumulator, the pressure-holding mode beingto hermetically close the servo chamber; (f) a first solenoid valvewhich switches between a pressure exerting mode and a non-pressureexerting mode, the pressure exerting mode being to exert the hydraulicpressure of the brake fluid stored in the accumulator on the spoolvalve, the non-pressure exerting mode being not to exert the hydraulicpressure of the brake fluid stored in the accumulator on the spoolvalve; and (g) a collision avoidance controller which works to determinewhether there is a risk of a collision of the vehicle equipped with thisbraking apparatus with an obstacle or not. When it is determined thatthere is the risk of the collision, the collision avoidance controlleropens the first solenoid valve to move the spool valve so as toestablish the pressure-increasing mode for creating the frictionalbraking force applied to the wheel.

When it is determined that there is the risk of collision with theobstacle, the collision avoidance controller opens the first solenoidvalve. This causes the hydraulic pressure of the brake fluid, asdelivered from the accumulator, to move the spool valve to a locationwhere the pressure-increasing mode is established, thereby developingthe braking force in the friction braking device. Basically, emergencybraking is achieved by installing the first solenoid valve toselectively exert the hydraulic pressure of the brake fluid stored inthe accumulator on the spool valve, thus allowing an emergency avoidancebraking system to be constructed with a minimum of equipment andfacilitating the mountability of the braking apparatus in the vehicles.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given hereinbelow and from the accompanying drawings of thepreferred embodiments of the invention, which, however, should not betaken to limit the invention to the specific embodiments but are for thepurpose of explanation and understanding only.

In the drawings:

FIG. 1 is a block diagram which illustrates a hybrid vehicle in which abraking apparatus according to an embodiment is mounted;

FIG. 2 is a partially longitudinal sectional view which illustrates thebraking apparatus of FIG. 1;

FIG. 3 is an enlarged view of a spool piston and a spool cylinder of ahydraulic booster of the braking apparatus of FIG. 2 in apressure-reducing mode;

FIG. 4 is a graph which represents a relation between a braking effortacting on a brake pedal and a braking force;

FIG. 5 is an enlarged view of a spool piston and a spool cylinder of ahydraulic booster of the braking apparatus of FIG. 2 in apressure-increasing mode;

FIG. 6 is an enlarged view of a spool piston and a spool cylinder of ahydraulic booster of the braking apparatus of FIG. 2 in apressure-holding mode;

FIG. 7 is a partially enlarged view of a rear portion of a hydraulicbooster of the braking apparatus of FIG. 2;

FIG. 8A is a longitudinal sectional view which illustrates an internalstructure of a first solenoid valve installed in the braking apparatusof FIG. 2;

FIG. 8B is a longitudinal sectional view which illustrates an internalstructure of a first solenoid valve installed in the braking apparatusof FIG. 2;

FIG. 9 is a flowchart of a collision avoidance control program to beexecuted by the braking apparatus of FIG. 1;

FIG. 10A is a view which represents a first braking map for use incontrolling a braking force in a collision avoidance braking mode;

FIG. 10B is a view which represents a second braking map for use incontrolling a braking force in a collision avoidance braking mode;

FIG. 10C is a view which represents a third braking map for use incontrolling a braking force in a collision avoidance braking mode;

FIG. 11 is a view which represents a relation between a speed of avehicle and a braking force in a collision avoidance braking mode of thebraking apparatus of FIG. 2;

FIG. 12 is a view which represents a relation among a vehicle equippedwith the braking apparatus of FIG. 2, an obstacle, a detectable range ofan obstacle detector, and a collision risk warning location;

FIG. 13 is an enlarged view of a spool piston and a spool cylinder of ahydraulic booster of the braking apparatus of FIG. 2 in a collisionavoidance braking mode;

FIG. 14 is a partially longitudinal sectional view which illustrates abraking apparatus according to the second embodiment;

FIG. 15A is a view which illustrates a relation between a braking forceand a rate of increase in braking force in the braking apparatuses ofFIGS. 2 and 14; and

FIG. 15B is a view which illustrates a relation between a braking forceand a rate of increase in braking force in a frictional brake unitequipped with a pressure regulator and non-linear electromagnetic typefirst and second solenoid valves.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, wherein like reference numbers refer to likeor equivalent parts in several views, particularly to FIG. 1, there isshown a brake system (i.e., a braking apparatus) B for vehicles such asautomobiles according to an embodiment. The drawings are merelyschematic views which do not necessarily illustrate dimensions of partsof the brake system B precisely.

Hybrid Vehicle

The brake system B, as referred to herein, is engineered as a frictionbrake unit mounted in a hybrid vehicle. The hybrid vehicle is equippedwith a hybrid system to drive wheels, for example, front left and rightwheels Wfl and Wfr. The hybrid vehicle also includes a brake ECU(Electronic Control Unit) 6, an engine ECU (Electronic Control Unit) 8,a hybrid ECU (Electronic Control Unit) 9, a hydraulic booster 10, apressure regulator 53, a hydraulic pressure generator 60, a brake pedal(i.e., a brake actuating member) 71, a brake sensor 72, a first solenoidvalve 91 (see FIG. 2), and a second solenoid valve 92 (see FIG. 2), anobstacle detector 97, a warning device 98, an internal combustion engine501, an electric motor 502, a power pushing member 40, a split device503, a power transmission device 504, an inverter 506, and a storagebattery 507.

The brake system B (i.e., the friction brake unit) is essentially madeup of the brake ECU 6, the hydraulic booster 10, the pressure regulator53, the hydraulic pressure generator 60, the brake pedal 71, the brakesensor 72, the first solenoid valve 91, the second solenoid valve 92,the obstacle detector 97, and the warning device 98.

The output power of the engine 501 is transmitted to the driven wheelsthrough the power split device 503 and the power transmission device504. The output power of the motor 502 is also transmitted to the drivenwheels through the power transmission device 504.

The inverter 506 works to achieve conversion of voltage between themotor 502 or an electric generator 505 and the battery 507. The engineECU 8 works to receive instructions from the hybrid ECU 9 to control thepower, as outputted from the engine 501. The hybrid ECU 9 serves tocontrol operations of the motor 502 and the generator 505 through theinverter 506. The hybrid ECU 9 is connected to the battery 507 andmonitors the state of charge (SOC) of and current charged in the battery507.

A combination of the generator 505, the inverter 506, and the battery507 makes a regenerative braking system A. The regenerative brakingsystem A works to make the wheel Wfl and Wfr produce a regenerativebraking force as a function of an actually producible regenerativebraking force, which will be described later in detail. The motor 502and the generator 505 are illustrated in FIG. 1 as being separate parts,but their operations may be achieved by a single motor/generator.

Friction braking devices Bfl, Bfr, Brl, and Brr are disposed near thewheels Wfl, Wfr, Wrl, and Wrr of the vehicle. The friction brakingdevice Bfl includes a brake disc DRfl and a brake pad (not shown). Thebrake disc DRfl rotates along with the wheel Wfl. The brake pad is of atypical type and pressed against the brake disc DRfl to produce afriction braking power. Similarly, the friction braking devices Bfr,Brl, and Brr are made up of brake discs DRfl, DRfr, DRrl, and DRrr andbrake pads (not shown), respectively, and identical in operation andstructure with the friction braking device Bfl. The explanation thereofin detail will be omitted here. The friction braking devices Bfl, Bfr,Brl, and Brr also include wheel cylinders WCfl, WCfr, WCri, and WCrr,respectively, which are responsive to a master pressure (which is alsocalled master cylinder pressure) that is hydraulic pressure, asdeveloped by the hydraulic booster 10, required to press the brake padsagainst the brake discs DRfl, DRfr, DRrl, and DRrr, respectively.

Wheel speed sensors Sfl, Sfr, Srl, and Srr are disposed adjacent thewheels Wfl, Wfr, Wrl, and Wrr of the vehicle. Each of the wheel speedsensors Sfl, Sfr, Srl, and Srr works to output a pulse signal of afrequency as a function of rotational speed of a corresponding one ofthe wheels Wfl, Wfr, Wrl, and Wrr to the brake ECU 6.

The brake sensor 72 measures the amount of stroke, or position of thebrake pedal 71 depressed by the vehicle operator or driver and outputs asignal indicative thereof to the brake ECU 6. The brake ECU 6 calculatesa braking force, as required by the vehicle driver, as a function of thesignal outputted from the brake sensor 72. The brake ECU 6 calculates atarget regenerative braking force as a function of the required brakingforce and outputs a signal indicative of the target regenerative brakingforce to the hybrid ECU 9. The hybrid ECU 9 calculates the actuallyproducible regenerative braking force as a function of the targetregenerative braking force and outputs a signal indicative thereof tothe brake ECU 6.

The acceleration sensor 96 is connected to the brake ECU 6. Theacceleration sensor 96 measures the degree of acceleration of thevehicle and outputs a signal indicative thereof to the brake ECU 6. Theobstacle detector 97 is implemented by a stereo camera, amillimeter-wave radar, or an infrared radar to detect an obstaclepresent ahead of the vehicle. The obstacle detector 97 is mounted infront of a driver's seat or a bumper of the vehicle and oriented forwardfrom the vehicle.

The brake ECU 6 analyzes the output from the obstacle detector 97 todetermine whether there is a probability that the vehicle equipped withthe brake system B (which will also be referred to as a system vehiclebelow) will collide with the obstacle or not. Specifically, the brakeECU 6 calculates the speed and acceleration of the system vehicle usingoutputs from the wheel speed sensors Sfl, Sfr, Srl, and Srr. Next, thebrake ECU 6 calculates the distance between the system vehicle and theobstacle using the output from the obstacle detector 97 and alsodetermines the speed and acceleration of the obstacle when it isdetermined that the obstacle tracked by the obstacle detector 97 is avehicle preceding the system vehicle.

Subsequently, the brake ECU 6 calculates the speed and acceleration ofthe system vehicle relative to the obstacle tracked by the obstacledetector 97. The brake ECU 6 analyzes the distance to the obstacle, therelative speed, and the relative acceleration of the system vehicle todecide whether there is a possibility that the system vehicle willcollide with the obstacle or not.

How to determine the probability of collision with an obstacle is taughtin, for example, Japanese Patent First Publication Nos. 2006-168629 and2012-192776, disclosures of which are incorporated herein by reference.

The warning device 98 is implemented by a speaker, a display, and/or awarning lamp and serves to inform the driver of the vehicle of the riskof collision with an obstacle.

Hydraulic Pressure Generator

The structure and operation of the hydraulic pressure generator 60 willbe described in detail with reference to FIG. 2. The hydraulic pressuregenerator 60 works to produce an accumulator pressure and includes anaccumulator 61, a hydraulic pressure pump 62, and a pressure sensor 65.

The accumulator 61 stores therein brake fluid under pressure.Specifically, the accumulator 61 stores accumulator pressure that is thehydraulic pressure of the brake fluid, as created by the hydraulicpressure pump 62. The accumulator 61 connects with the pressure sensor65 and the hydraulic pressure pump 62 through a pipe 66. The hydraulicpressure pump 62 connects with a reservoir 19. The hydraulic pressurepump 62 is driven by an electric motor 63 to deliver the brake fluidfrom the reservoir 19 to the accumulator 61.

The pressure sensor 65 works to measure the accumulator pressure that isthe pressure in the accumulator 61. When the accumulator pressure isdetermined through the pressure sensor 65 to have dropped below a givenvalue, the brake ECU 6 outputs a control signal to actuate the motor 63.

Hydraulic Booster

The structure and operation of the hydraulic booster 10 will bedescribed below with reference to FIG. 2. The hydraulic booster 10 worksas a pressure generator to regulate the accumulator pressure, asdeveloped by the hydraulic pressure generator 60, as a function of thestroke of (i.e., a driver's effort on) the brake pedal 71 to produce aservo pressure which is, in turn, used to generate the master pressure.

The hydraulic booster 10 includes a master cylinder 11, a fail-safecylinder 12, a first master piston 13, a second master piston 14, aninput piston 15, an operating rod 16, a first return spring 17, a secondreturn spring 18, a reservoir 19, a stopper 21, a mechanical reliefvalve 22, a spool piston 23, a spool cylinder 24, a spool spring 25, asimulator spring 26, a pedal return spring 27, a movable member 28, afirst spring retainer 29, a second spring retainer 30, a connectingmember 31, a movable member 32, a retaining piston 33, a simulatorrubber 34 serving as a cushion, a spring retainer 35, a fail-safe spring36, a damper 37, a first spool spring retainer 38, a second springretainer 39, a pushing member 40, and sealing members 41 to 49.

In the following discussion, a part of the hydraulic booster 10 wherethe first master piston 13 is disposed will be referred to as the frontof the hydraulic booster 10, while a part of the hydraulic booster 10where the operating rod 16 is disposed will be referred to as the rearof the hydraulic booster 10. An axial direction (i.e., a lengthwisedirection) of the hydraulic booster 10, thus, represents a front-backdirection of the hydraulic booster 10.

The master cylinder 11 is of a hollow cylindrical shape which has abottom 11 a on the front of the hydraulic booster 10 and an openingdefining the rear of the hydraulic booster 10. The master cylinder 11has a given length aligned with the length of the hydraulic booster 10,a front end (i.e. the bottom 11 a), and a rear end (i.e., the opening)at the rear of the hydraulic booster 10. The master cylinder 11 also hasa cylindrical cavity 11 p extending in the lengthwise or longitudinaldirection thereof. The master cylinder 11 is installed in the vehicle.The master cylinder 11 has a first port 11 b, a second port 11 c, athird port 11 d, a fourth port 11 e, a fifth port 11 f (i.e., a supplyport), a sixth port 11 g, and a seventh port 11 h all of whichcommunicate with the cylindrical cavity 11 p and which are arranged inthat order from the front to the rear of the master cylinder 11. Thesecond port 11 c, the fourth port 11 e, the sixth port 11 g, and theseventh port 11 h connect with the reservoir 19 in which the brake fluidis stored. The reservoir 19, thus, communicates with the cylindricalcavity 11 p of the master cylinder 11.

The sealing members 41 and 42 are disposed in annular grooves formed inan inner peripheral wall of the master cylinder 11 across the secondport 11 c. The sealing members 41 and 42 are in hermetic contact with anentire outer circumference of the first master piston 13. Similarly, thesealing members 43 and 44 are disposed in annular grooves formed in theinner peripheral wall of the master cylinder 11 across the fourth port11 e. The sealing members 43 and 44 are in hermetic contact with anentire outer circumference of the second master piston 14.

The sealing members 45 and 46 are disposed in annular grooves formed inthe inner peripheral wall of the master cylinder 11 across the fifthport 11 f. The sealing members 45 and 46 are in hermetic contact withentire outer circumferences of a first cylindrical portion 12 b and asecond cylindrical portion 12 c of the fail-safe cylinder 12, as will bedescribed later in detail. The sealing member 47 is disposed in anannular groove formed in the inner peripheral wall of the mastercylinder 11 behind the sealing member 46 in hermetic contact with theentire outer circumference of the second cylindrical portion 12 c.Similarly, the sealing members 48 and 49 are disposed in annular groovesformed in the inner peripheral wall of the master cylinder 11 across theseventh port 11 h. The sealing members 48 and 49 are in hermetic contactwith the entire outer circumference of the second cylindrical portion 12c of the fail-safe cylinder 12.

A support member 59 is disposed on the front surface of the sealingmember 45. The sealing member 45 and the support member 59 are installedin a common retaining groove 11 j formed in the inner wall of the mastercylinder 11. The sealing member 45 and the support member 59 are, asclearly illustrated in FIG. 3, placed in abutment contact with eachother. The support member 59 is of a ring shape and has a slit 59 aformed therein. The support member 59 is made of elastic material suchas resin and has, as illustrated in FIG. 3, an inner peripheral surfacein contact with the outer circumferential surface of the firstcylindrical portion 12 b of the fail-safe cylinder 12 which will bedescribed later in detail.

Referring back to FIG. 2, the fifth port 11 f works as a supply portwhich establishes a fluid communication between the outer periphery ofthe master cylinder 11 and the cylindrical cavity 11 p. The fifth port11 f connects with the accumulator 61 through a pipe 67 defining a flowpath. In other words, the accumulator 61 communicates with thecylindrical cavity 11 p of the master cylinder 11, so that theaccumulator pressure is supplied to the fifth port 11 f.

The fifth port 11 f and the sixth port 11 g communicate with each otherthrough a connecting fluid path 11 k in which a mechanical relief valve22 is mounted. The mechanical relief valve 22 works to block a flow ofthe brake fluid from the sixth port 11 g to the fifth port 11 f andallow a flow of the brake fluid from the fifth port 11 f to the sixthport 11 g when the pressure in the fifth port 11 f rises above a givenlevel.

The first master piston 13 is disposed in a front portion of thecylindrical cavity 11 p of the master cylinder 11, that is, locatedbehind the bottom 11 a, so that it is slidable in the longitudinaldirection of the cylindrical cavity 11 p. The first master piston 13 isof a bottomed cylindrical shape and made up of a hollow cylindricalportion 13 a and a cup-shaped retaining portion 13 b extending behindthe cylindrical portion 13 a. The retaining portion 13 b is fluidicallyisolated from the cylindrical portion 13 a. The cylindrical portion 13 ahas fluid holes 13 c formed therein. The cylindrical cavity 11 pincludes a first master chamber 10 a located in front of the retainingportion 13 b. Specifically, the first master cylinder 10 a is defined bythe inner wall of the master cylinder 11, the cylindrical portion 13 a,and the retaining portion 13 b. The first port 11 b communicates withthe first master chamber 10 a. The first master chamber 10 a is filledwith the brake fluid which is supplied to the wheel cylinders WCfl,WCfr, WCrl, and WCrr.

The first return spring 17 is disposed between the bottom 11 a of themaster cylinder 11 and the retaining portion of the first master piston13. The first return spring 17 urges the first master piston 13 backwardto place the first master piston 13 at an initial position, asillustrated in FIG. 2, unless the brake pedal 71 is depressed by thevehicle driver.

When the first master piston 13 is in the initial position, the secondport 11 c coincides or communicates with the fluid holes 13 c, so thatthe reservoir 19 communicates with the first master chamber 10 a. Thiscauses the brake fluid to be delivered from the reservoir 19 to thefirst master chamber 10 a. An excess of the brake fluid in the firstmaster chamber 10 a is returned back to the reservoir 19. When the firstmaster piston 13 travels frontward from the initial position, it willcause the second port 11 c to be blocked by the cylindrical portion 13a, so that the first master chamber 10 a is closed hermetically tocreate the master pressure therein.

The second master piston 14 is disposed in a rear portion of thecylindrical cavity 11 p of the master cylinder 11, that is, locatedbehind the first master piston 13, so that it is slidable in thelongitudinal direction of the cylindrical cavity 11 p. The second masterpiston 14 is made up of a first cylindrical portion 14 a, a secondcylindrical portion 14 b lying behind the first cylindrical portion 14a, and a retaining portion 14 c formed between the first and secondcylindrical portions 14 a and 14 b. The retaining portion 14 cfluidically isolates the first and second cylindrical portions 14 a and14 b from each other. The first cylindrical portion 14 a has fluid holes14 d formed therein.

The cylindrical cavity 11 p includes a second master chamber 10 blocated in front of the retaining portion 14 b. Specifically, the secondmaster cylinder 10 b is defined by the inner wall of the master cylinder11, the first cylindrical portion 14 a, and the retaining portion 14 c.The third port 11 d communicates with the second master chamber 10 b.The second master chamber 10 b is filled with the brake fluid which issupplied to the wheel cylinders WCfl, WCfr, WCrl, and WCrr. The secondmaster chamber 10 b defines a master chamber within the cylindricalcavity 11 p along with the first master chamber 10 a.

The second return spring 18 is disposed between the retaining portion 13of the first master piston 13 and the retaining portion 14 c of thesecond master piston 14. The second return spring 18 is greater in setload than the first return spring 17. The second return spring 18 urgesthe second master piston 14 backward to place the second master piston14 at an initial position, as illustrated in FIG. 2, unless the brakepedal 71 is depressed by the vehicle driver.

When the second master piston 14 is in the initial position, the fourthport 11 e coincides or communicates with the fluid holes 14 d, so thatthe reservoir 19 communicates with the second master chamber 10 b. Thiscauses the brake fluid to be delivered from the reservoir 19 to thesecond master chamber 10 b. An excess of the brake fluid in the secondmaster chamber 10 b is returned back to the reservoir 19. When thesecond master piston 14 travels frontward from the initial position, itwill cause the fourth port 11 e to be blocked by the cylindrical portion14 a, so that the second master chamber 10 b is closed hermetically tocreate the master pressure therein.

The fail-safe cylinder 12 is disposed behind the second master piston 14within the cylindrical cavity 11 p of the master cylinder 11 to beslidable in the longitudinal direction of the cylindrical cavity 11 p.The fail-safe cylinder 12 is made up of the front cylindrical portion 12a, the first cylindrical portion 12 b, and the second cylindricalportion 12 c which are aligned with each other in the lengthwisedirection thereof. The front cylindrical portion 12 a, the firstcylindrical portion 12 b, and the second cylindrical portion 12 c areformed integrally with each other and all of a hollow cylindrical shape.The front cylindrical portion 12 a has an outer diameter a. The firstcylindrical portion 12 b has an outer diameter b which is greater thanthe outer diameter a of the front cylindrical portion 12 a. The secondcylindrical portion 12 c has an outer diameter c which is greater thanthe outer diameter b of the first cylindrical portion 12 b. Thefail-safe cylinder 12 has an outer shoulder formed between the frontcylindrical portion 12 a and the first cylindrical portion 12 b todefine a pressing surface 12 i.

The second cylindrical portion 12 c has a flange 12 h extending outwardfrom a rear end thereof. The flange 12 h contacts with the stopper 21 tostop the fail-safe cylinder 12 from moving outside the master cylinder11. The second cylindrical portion 12 c has a rear end formed to begreater in inner diameter than another portion thereof to define aninner shoulder 12 j.

The front cylindrical portion 12 a is disposed inside the secondcylindrical portion 14 b of the second master piston 14. The firstcylindrical portion 12 b has first inner ports 12 d formed in a rearportion thereof. The first inner ports 12 d communicate between theouter peripheral surface and the inner peripheral surface of the firstcylindrical portion 12 b, in other words, passes through the thicknessof the first cylindrical portion 12 b. The second cylindrical portion 12c has formed in a front portion thereof a second inner port 12 e and athird inner port 12 f which extend through the thickness of the secondcylindrical portion 12 c. The second cylindrical portion 12 c also hasfourth inner ports 12 g formed in a middle portion thereof. The fourthinner ports 12 g extend through the thickness of the second cylindricalportion 12 c and opens toward the front end (i.e., the head) of theinput piston 15 disposed within the fail-safe cylinder 12.

The second cylindrical portion 12 c, as illustrated in FIG. 3, has astopper 12 m formed on a front inner peripheral wall thereof. Thestopper 12 m has formed therein fluid flow paths 12 n extending in thelongitudinal direction of the second cylindrical portion 12 c.

The input piston 15 is, as clearly illustrated in FIG. 2, located behindthe spool cylinder 24 and the spool piston 23, which will be describedlater in detail, to be slidable in the longitudinal direction thereofwithin a rear portion of the second cylindrical portion 12 c of thefail-safe cylinder 12 (i.e., the cylindrical cavity 11 p). The inputpiston 15 is made of a cylindrical member and substantially circular incross section thereof. The input piston 15 has a rod-retaining chamber15 a formed in a rear end thereof. The rod-retaining chamber 15 a has aconical bottom. The input piston 15 also has a spring-retaining chamber15 b formed in a front end thereof. The input piston 15 has an outershoulder 15 e to have a small-diameter rear portion which is smaller inouter diameter than a major portion thereof.

The input piston 15 has seal retaining grooves (i.e., recesses) 15 c and15 d formed in an outer periphery thereof. Sealing members 55 and 56 aredisposed in the seal retaining grooves 15 c and 15 d in hermeticalcontact with an entire inner circumference of the second cylindricalportion 12 c of the fail-safe cylinder 12.

The input piston 15 is coupled with the brake pedal 71 through theoperating rod 16 and a connecting member 31, so that the effort actingon the brake pedal 71 is transmitted to the input piston 15. The inputpiston 15 works to transmit the effort, as exerted thereon, to the spoolpiston 23 through the simulator spring 26, the movable member 32, thesimulator rubber 34, the retaining piston 33, and the damper 37, so thatthe spool piston 23 travels in the longitudinal direction thereof.

Referring to FIG. 7, the spring retainer 35 is made up of a hollowcylinder 35 a and a ring-shaped support 35 b extending inwardly from afront edge of the hollow cylinder 35 a. The spring retainer 35 is fit inthe rear end of the second cylindrical portion 12 c with the support 35b having the front surface thereof placed in contact with the shoulder15 e of the input piston 15.

The stopper 21 is attached to the inner wall of the rear end of themaster cylinder 11 to be movable. The stopper 21 is designed as astopper plate and made up of a ring-shaped base 21 a, a hollow cylinder21 b, and a stopper ring 21 c. The hollow cylinder 21 b extends forwardfrom the front end of the base 21 a. The stopper ring 21 c extendsinwardly from the front end of the hollow cylinder 21 b.

The base 21 a has a front surface 21 d which lies inside the hollowcylinder 21 b as a support surface with which the rear end (i.e., theflange 12 h) of the fail-safe cylinder 12 is placed in contact. Theflange 12 h will also be referred to as a contact portion below. Thestopper 21 also includes a ring-shaped retaining recess 21 f formed inthe front surface of the base 21 a inside the support surface 21 d inthe shape of a groove. Within the retaining recess 21 f, the rear end ofthe cylinder 35 a of the spring retainer 35 is fit. The stopper 21further includes a ring-shaped protrusion 21 g extending from the frontof the base 21 a inside the retaining recess 21 f.

The base 21 a has a domed recess 21 e formed on a central area of therear end thereof. The recess 21 e serves as a seat and is of an arc orcircular shape in cross section. The recess 21 e will also be referredto as a seat below. The master cylinder 11 has a C-ring 86 fit in agroove formed in the inner wall of the open rear end thereof. The C-ring86 works as a stopper to hold the stopper 21 from being removed from themaster cylinder 11.

The movable member 28 is used as a spacer and made of a ring-shapedmember. The movable member 28 has a front surface which is orientedtoward the front of the master cylinder 11 and defines a convex ordome-shaped pressing surface 28 a. The pressure surface 28 a is of anarc or circular shape in cross section. The pressing surface 28 a iscontoured to conform with the shape of the seat 21 e. The movable member28 is disposed on the front end of the first spring retainer 29 whichfaces the front of the master cylinder 11. The movable member 28 is alsoarranged behind the stopper 21 with the pressing surface 28 a beingplaced in slidable contact with the seat 21 e. The movable member 28 ismovable or slidable on the stopper 21 (i.e., the seat 21 e).

The fail-safe spring 36 is disposed between the support 35 b of thespring retainer 35 and the protrusion 21 g of the stopper 21 within thecylinder 35 a of the spring retainer 35. The fail-safe spring 36 is madeup of a plurality of diaphragm springs and works to urge the fail-safecylinder 12 forward against the master cylinder 11.

The first spring retainer 29 is made up of a hollow cylinder 29 a and aflange 29 b extending from the front end of the hollow cylinder 29 ainwardly and outwardly. The first spring 29 is arranged behind themovable member 28 with the flange 29 b placed in abutment contact withthe rear end of the movable member 28.

The operating rod 16 has a pressing ball 16 a formed on the front endthereof and a screw 16 b formed on the rear end thereof. The operatingrod 16 is joined to the rear end of the input piston 15 with thepressing ball 16 a fit in the rod-retaining chamber 15 a. The operatingrod 16 has a given length extending in the longitudinal direction of thehydraulic booster 10. Specifically, the operating rod 16 has the lengthaligned with the length of the hydraulic booster 10. The operating rod16 passes through the movable member 28 and the first spring retainer29.

The second spring retainer 30 is disposed behind the first springretainer 29 in alignment therewith and secured to the rear portion ofthe operating rod 16. The second spring retainer 30 is of a hollowcylindrical shape and made up of an annular bottom 30 a and a cylinder30 b extending from the bottom 30 a frontward. The bottom 30 a has athreaded hole 30 c into which the screw 16 b of the operating rod 16 isfastened.

The pedal return spring 27 is disposed between the flange 29 b of thefirst spring retainer 29 and the bottom 30 a of the second springretainer 30. The pedal return spring 27 is held inside the cylinder 29 aof the first spring retainer 29 and the cylinder 30 b of the secondspring retainer 30.

The connecting member 31 has a threaded hole 31 a formed in the frontend thereof. The screw 16 b of the operating rod 16 is fastened into thethreaded hole 31 a to join the connecting member 31 to the rear end ofthe operating rod 16. The bottom 30 a of the second spring retainer 30is in contact with the front end of the connecting member 31. Theconnecting member 31 has an axial through hole 31 b formed insubstantially the center thereof in the longitudinal direction of thehydraulic booster 10. The threaded hole 30 c of the second springretainer 30 and the threaded hole 31 a of the connecting member 31 arein engagement with the screw 16 b of the operating rod 16, therebyenabling the connecting member 31 to be regulated in position thereofrelative to the operating rod 16 in the longitudinal direction of theoperating rod 16.

The brake pedal 71 works as a brake actuating member and is made of alever on which an effort is exerted by the driver of the vehicle. Thebrake pedal 71 has an axial hole 71 a formed in the center thereof and amount hole 71 b formed in an upper portion thereof. A bolt 81 isinserted into the mount hole 71 b to secure the brake pedal 71 to amount base of the vehicle, as indicated by a broken line in FIG. 2. Thebrake pedal 71 is swingable about the bolt 81. A connecting pin 82 isinserted into the axial hole 71 a of the brake pedal 71 and the axialhole 31 b of the connecting member 31, so that the swinging motion ofthe brake pedal 71 is converted into linear motion of the connectingmember 31.

The pedal return spring 27 urges the second spring retainer 30 and theconnecting member 31 backward to keep the brake pedal at the initialposition, as illustrated in FIG. 2. The depression of the brake pedal 71will cause the brake pedal 71 to swing about the mount hole 71 b (i.e.,the bolt 81) and also cause the axial holes 71 a and 31 b to swing aboutthe mount hole 71 b. A two-dot chain line in FIG. 2 indicates a path oftravel of the axial holes 71 a and 31 b. Specifically, when the brakepedal 71 is depressed, the axial holes 71 a and 31 b move upward alongthe two-dot chain line. This movement causes the movable member 28 andthe first spring retainer 29 to swing or slide on the stopper 21 toprevent an excessive pressure (i.e., shearing force) from acting on thepedal return spring 27.

The retaining piston 33 is, as clearly illustrated in FIG. 2, disposedinside the front portion of the second cylindrical portion 12 c of thefail-safe cylinder 12 (i.e., within the cylindrical cavity 11 p of themaster cylinder 11) to be slidable in the longitudinal directionthereof. The retaining piston 33 is made of a bottomed cylindricalmember and includes a front end defining a bottom 33 a and a cylinder 33b extending rearward from the bottom 33 a The bottom 33 a has formed inthe front end thereof a concave recess 33 c serving as a retainingcavity. The cylinder 33 b has a seal-retaining groove 33 d formed in theouter circumference thereof. A seal 75 is fit in the seal-retaininggroove 33 d in contact with an entire inner circumference of the secondcylindrical portion 12 c of the fail-safe cylinder 12. A C-ring 85 is,as clearly illustrated in FIG. 2, fit in an angular groove formed in theouter periphery of the second cylindrical portion 12 c. The C-ring 85 iscontactable with the rear end of the retaining position 33, that is,works as a stopper to hold the retaining piston 33 from slidingbackward.

The movable member 32 is, as illustrated in FIG. 2, disposed inside therear portion of the second cylindrical portion 12 c of the fail-safecylinder 12 (i.e., within the cylindrical cavity 11 p of the mastercylinder 11) to be slidable in the longitudinal direction thereof. Themovable member 32 is made up of a flange 32 a formed on the front endthereof and a shaft 32 b extending backward from the flange 32 a in thelongitudinal direction of the hydraulic booster 10.

The flange 32 a has a rubber-retaining chamber 32 c formed in the frontend thereof in the shape of a concave recess. In the rubber-retainingchamber 32 c, the cylindrical simulator rubber 34 is fit which protrudesoutside the front end of the rubber-retaining chamber 32 c. When placedat an initial position, as illustrated in FIG. 2, the simulator rubber(i.e., the movable member 32) is located away from the retaining piston33.

The flange 32 a has formed therein a fluid path 32 h which communicatesbetween a fluid chamber formed in front of the movable member 32, inother words, between the front end of the flange 32 a and the inner wallof the retaining piston 33, and a simulator chamber 10 f, which will bedescribed later in detail. When the movable member 32 moves relative tothe retaining piston 33, it will cause the brake fluid to flow from theabove fluid chamber to the simulator chamber 10 f or vice versa, therebyfacilitating the sliding movement of the movable member 32 towards oraway from the retaining piston 33.

The simulator chamber 10 f (which will also be referred to as a strokechamber below) is defined by the inner wall of the second cylindricalportion 12 c of the fail-safe cylinder 12, the rear end of the retainingpiston 33, and the front end of the input piston 15. The simulatorchamber 10 f is filled with the brake fluid and works as a brakesimulator chamber to develop a reactive pressure in response to thebraking effort on the brake pedal 71.

The simulator spring 26 is a braking simulator member engineered as abraking operation simulator and disposed between the flange 32 a of themovable member 32 and the spring-retaining chamber 15 b of the inputpiston 15 within the simulator chamber 10 f. In other words, thesimulator spring 26 is located ahead of the input piston 15 within thesecond cylindrical portion 12 c of the fail-safe cylinder 12 (i.e., thecylindrical cavity 11 p of the master cylinder 11). The shaft 32 b ofthe movable member 32 is inserted into the simulator spring 26 to retainthe simulator spring 26. The simulator spring 26 has a front portionpress-fit on the shaft 32 b of the movable member 32. With thesearrangements, when the input piston 15 advances further from where thesimulator rubber 34 (i.e., the movable member 32) hits the retainingpiston 33, it will cause the simulator spring 26 to urge the inputpiston 15 backward.

The first inner ports 12 d open at the outer periphery of the firstcylindrical portion 12 b of the fail-safe cylinder 12. The secondcylindrical portion 12 c is, as described above, shaped to have theouter diameter c greater than the outer diameter b of the firstcylindrical portion 12 b. Accordingly, the exertion of the accumulatorpressure on the fifth port 11 f (i.e., when the brake fluid is beingsupplied from the accumulator 61 to the fifth port 11 f) will causeforce or hydraulic pressure, as created by the accumulator pressure(i.e., the pressure of the brake fluid delivered from the accumulator61) and a difference in traverse cross-section between the firstcylindrical portion 12 b and the second cylindrical portion 12 c, topress the fail-safe cylinder 12 rearward against the stopper 21, therebyplacing the fail-safe cylinder 12 at a rearmost position (i.e., theinitial position) of the above describe preselected allowable range.

When the fail-safe cylinder 12 is in the initial position, the fourthinner ports 12 g communicate with the seventh port 11 h of the mastercylinder 11. Specifically, the hydraulic communication between thesimulator chamber 10 f and the reservoir 19 is established by areservoir flow path, as defined by the fourth inner ports 12 g and theseventh port 11 h. The simulator chamber 10 f is a portion of thecylindrical cavity 11 p, as defined ahead the input piston 15 inside thefail-safe cylinder 12. A change in volume of the simulator chamber 10 farising from the longitudinal sliding movement of the input piston 15causes the brake fluid within the simulator chamber 10 f to be returnedback to the reservoir 19 or the brake fluid to be supplied from thereservoir 19 to the simulator chamber 10 f, thereby allowing the inputpiston 15 to move frontward or backward in the longitudinal directionthereof without undergoing any hydraulic resistance.

The spool cylinder 24 is, as illustrated in FIG. 3, fixed in the firstcylindrical portion 12 b of the fail-safe cylinder 12 (i.e., thecylindrical cavity 11 p of the master cylinder 11) behind the secondmaster piston 14. The spool cylinder 24 is of a substantially hollowcylindrical shape. The spool cylinder 24 has seal-retaining grooves 24 aand 24 b formed in an outer periphery thereof in the shape of a concaverecess. Sealing members 57 and 58 are fit in the seal-retaining grooves24 a and 24 b in direct contact with an entire circumference of theinner wall of the first cylindrical portion 12 b to create a hermeticalseal therebetween. The sealing members 57 and 58 develop mechanicalfriction between themselves and the inner wall of the first cylindricalportion 12 b to hold the spool cylinder 24 from advancing in the firstcylindrical portion 12 b. The spool cylinder 24 has the rear end placedin contact with the stopper 12 m, so that it is held from movingbackward.

The spool cylinder 24 has formed therein a spool port 24 c whichcommunicates between inside and outside thereof. The spool port 24 ccommunicates with the first inner ports 12 d. The spool cylinder 24 hasa first spool groove 24 d formed in a portion of an inner wall thereofwhich is located behind the spool port 24 c. The first spool groove 24 dextends along an entire inner circumference of the spool cylinder 24 inthe shape of a concave recess. The spool cylinder 24 also has a secondspool groove 24 f formed in a rear end of the inner wall thereof whichis located behind the first spool groove 24 d. The second spool groove24 f extends along the entire inner circumference of the spool cylinder24 in the shape of a concave recess.

The spool cylinder 24 also has a fluid flow groove 24 e formed in aportion of an outer wall thereof which is located behind theseal-retaining groove 24 b. The fluid flow groove 24 e extends along anentire outer circumference of the spool cylinder 24 in the shape of aconcave recess. The third inner port 12 f opens into the fluid flowgroove 24 e. Specifically, the fluid flow groove 24 e defines a flowpath leading to the reservoir 19 through the third inner port 12 f andthe sixth port 11 g.

The spool piston 23 is made of a cylindrical shaft which is of acircular cross section. The spool piston 23 is disposed inside the spoolcylinder 24 to be slidable in the longitudinal direction thereof.

The spool piston 23 has a cylindrical rear end defining an in-cylinderportion 23 a which is slidable in the longitudinal direction thereofinto and out of the retaining cavity 33 c of the retaining piston 33.The outer periphery of the in-cylinder portion 23 a of the spool piston23 is separate from the inner periphery of the retaining cavity 33 c ofthe retaining piston 33 through an air gap to define a pilot-pressureinduction path 23 x.

The damper 37 is installed between the bottom of the retaining cavity 33c and the rear end of the spool piston 23. The damper 37 is made of acylindrical elastic rubber, but may alternatively be implemented by anelastically deformable member such as a coil spring or a diaphragm.

The spool piston 23 has a third spool groove 23 b formed in an axialcentral portion of an outer wall thereof. The third spool groove 23 bextends along an entire outer circumference of the spool piston 23 inthe shape of a concave recess. The spool piston 23 also has a fourthspool groove 23 c formed in a portion of the outer wall thereof which islocated behind the third spool groove 23 b. The fourth spool groove 23 cextends along the entire outer circumference of the spool piston 23 inthe shape of a concave recess. The spool piston 23 also has an elongatedfluid flow hole 23 e which extends along the longitudinal center linethereof from the front end behind the middle of the length of the spoolpiston 23. The spool piston 23 also has formed therein a first fluidflow port 23 d and a second fluid flow port 23 f which communicatebetween the fourth spool groove 23 c and the fluid flow hole 23 e.

Referring back to FIG. 2, the hydraulic booster 10 also includes a servochamber 10 c which is defined by the rear inner wall of the secondmaster piston 14, the front end portion of the spool piston 23, and thefront end of the spool cylinder 24 behind the retaining portion 14 c ofthe second master piston 14 within the cylindrical cavity 11 p of themaster cylinder 11.

The first spool spring retainer 38 is, as clearly illustrated in FIG. 2,made up of a retaining disc 38 a and a cylindrical fastener 38 b. Theretaining disc 38 a is fit in an inner front end wall of the frontcylindrical portion 12 a of the fail-safe cylinder 12 and closes a frontopening of the front cylindrical portion 12 a. The cylindrical fastener38 b extends frontward from the front center of the retaining disc 38 a.The cylindrical fastener 38 b has an internal thread formed in an innerperiphery thereof. The retaining disc 38 a has a contact portion 38 cformed on a central area of the rear end thereof. The retaining disc 38a also has fluid flow holes 38 d passing through the thickness thereof.

The pushing member 40 is made of a rod and has a rear end engaging theinternal thread of the cylindrical fastener 38 b.

The second spool spring retainer 39 is, as illustrated in FIG. 3, madeup of a hollow cylindrical body 39 a and a ring-shaped retaining flange39 b The cylindrical body 39 a has a front end defining a bottom 39 c.The retaining flange 39 b extends radially from the rear end of thecylindrical body 39 a. The front end of the spool piston 23 is fit inthe cylindrical body 39 a in engagement with an inner periphery of thecylindrical body 39 a, so that the second spool spring retainer 39 issecured to the front end of the spool piston 23. The bottom 39 c has athrough hole 39 d formed therein. The second spool spring retainer 39is, as can be seen from FIG. 2, aligned with the first spool springretainer 38 at a given interval away from the contact portion 38 c.

The spool spring 25 is, as illustrated in FIGS. 2 and 3, disposedbetween the retaining disc 38 a of the first spool spring retainer 38and the retaining flange 39 b of the second spool spring retainer 39.The spool spring 25 works to urge the spool piston 23 backward relativeto the fail-safe cylinder 12 (i.e., the master cylinder 11) and thespool cylinder 24.

The spring constant of the simulator spring 26 is set greater than thatof the spool spring 25. The spring constant of the simulator spring 26is also set greater than that of the pedal return spring 27.

Simulator

The simulator made up of the simulator spring 26, the pedal returnspring 27, and the simulator rubber 34 will be described below. Thesimulator is a mechanism engineered to apply a reaction force to thebrake pedal 71 to imitate an operation of a typical brake system, thatis, make the driver of the vehicle experience the sense of depression ofthe brake pedal 71.

When the brake pedal 71 is depressed, the pedal return spring 27contracts, thereby creating a reaction pressure (which will also bereferred to as a reactive force) acting on the brake pedal 71. Thereaction pressure is given by the sum of a set load of the pedal returnspring 27 and a product of the spring constant of the pedal returnspring 27 and the stroke of the brake pedal 71 (i.e., the connectingmember 31).

When the brake pedal 71 is further depressed, and the simulator rubber34 hits the retaining piston 33, the pedal return spring 27 and thesimulator spring 26 contract. The reaction pressure acting on the brakepedal is given by a combination of physical loads generated by thesimulator spring 26 and the pedal return spring 27. Specifically, a rateof increase in reaction pressure exerted on the brake pedal 71 duringthe stroke of the brake pedal 71 (i.e., unit of depression of the brakepedal 71) after the simulator rubber 34 contacts the retaining piston 33will be greater than that before the simulator rubber 34 contacts theretaining piston 33.

When the simulator rubber 34 contacts the retaining piston 33, and thebrake pedal 71 is further depressed, it usually causes the simulatorrubber 34 to contract. The simulator rubber 34 has a spring constantwhich increases, in the nature thereof, as the simulator rubber 34contracts. Therefore, there is a transient time for which the reactionpressure exerted on the brake pedal 71 changes gently to minimize thedriver's discomfort arising from a sudden change in reaction pressureexerted on the foot of the driver of the vehicle.

Specifically, the simulator rubber 34 serves as a cushion to decreasethe rate of change in reaction pressure acting on the brake pedal 71during the depression thereof. The simulator rubber 34 of thisembodiment is, as described above, secured to the movable member 32, butmay be merely placed between opposed end surfaces of the movable member32 and the retaining piston 33. The simulator rubber 34 mayalternatively be attached to the rear end of the retaining piston 33.

As described above, the reaction pressure exerted on the brake pedal 71during the depression thereof increases at a smaller rate until thesimulator rubber 34 contacts the retaining piston and then increases ata greater rate, thereby giving a typical sense of operation (i.e.,depression) of the brake pedal 71 to the driver of the vehicle.

Pressure Regulator

The pressure regulator 53 works to increase or decrease the masterpressure that is the pressure of brake fluid delivered from the masterchambers 10 a and 10 b to produce wheel cylinder pressure to be fed tothe wheel cylinders WCfl, WCfr, WCrl, and WCrr and is engineered toachieve known anti-lock braking control or known electronic stabilitycontrol to avoid lateral skid of the vehicle. The pressure regulator 53may be designed to have a known structure such as taught in JapanesePatent First Publication No. 2013-6534 or No. 2008-87069, andexplanation thereof in detail will be omitted here. The wheel cylindersWCfr and WCfl are connected to the first port 11 b of the first mastercylinder 10 a through the pipe 52 and the pressure regulator 53.Similarly, the wheel cylinders WCrr and WCrl are connected to the thirdport 11 d of the second master cylinder 10 b through the pipe 51 and thepressure regulator 53.

Operation of Hydraulic Booster

The operation of the hydraulic booster 10 will be described below indetail. The hydraulic booster 10 is equipped with a spool valve made upof the spool cylinder 24 and the spool piston 23. Upon depression of thebrake pedal 71, the spool valve is driven or slides in the longitudinaldirection of the master cylinder 11 in response to the driver's efforton the brake pedal 71. The hydraulic booster 10 then enters any one ofthe pressure-reducing mode, the pressure-increasing mode, and thepressure-holding mode.

Pressure-Reducing Mode

The pressure-reducing mode is entered when the brake pedal 71 is notdepressed or the driver's effort (which will also be referred to asbraking effort below) on the brake pedal 71 is lower than or equal to africtional braking force generating level P2, as indicated in a graph ofFIG. 4. When the brake pedal is, as illustrated in FIG. 2, released, sothat the pressure-reducing mode is entered, the simulator rubber 34(i.e., the movable member 32) is separate from the bottom 33 a of theretaining piston 33.

When the simulator rubber 34 is located away from the bottom 33 a of theretaining piston 33, the spool piston 23 is placed by the spool spring25 at the rearmost position in the movable range thereof (which willalso be referred to as a pressure-reducing position below). The spoolport 24 c is, as illustrated in FIG. 3, blocked by the outer peripheryof the spool piston 23, so that the accumulator pressure that is thepressure in the accumulator 61 is not exerted on the servo chamber 10 c.

The fourth spool groove 23 c of the spool piston 23, as illustrated inFIG. 3, communicates with the second spool groove 24 f of the spoolcylinder 24. The servo chamber 10 c, therefore, communicates with thereservoir 19 through a pressure-reducing flow path, as defined by thefluid flow hole 23 e, the first fluid flow part 23 d, the fourth spoolgroove 23 c, the second spool groove 24 f, the fluid flow path 12 n, thefluid flow groove 24 e, the third inner port 12 f, and the sixth port 11g. This causes the pressure in the servo chamber 10 c to be equal to theatmospheric pressure, so that the master pressure is not developed inthe first master chamber 10 a and the second master chamber 10 b.

When the brake pedal 71 is depressed, and the simulator rubber 34touches the bottom 33 a of the retaining piston 33 to develop thepressure (which will also be referred to as an input pressure below)urging the spool piston 23 forward through the retaining piston 33, butsuch pressure is lower in level than the pressure, as produced by thespool spring 25 and exerted on the spool piston 23, the spool piston 23is kept from moving forward in the pressure-reducing position. Note thatthe above described input pressure exerted on the spool piston 23through the retaining piston 33 is given by subtracting a load requiredto compress the pedal return spring 27 from a load applied to theconnecting member 31 upon depression of the brake pedal 71. When theload or effort applied to the brake pedal 71 is lower than or equal tothe frictional braking force generating level P2, the hydraulic booster10 is kept from entering the pressure-increasing mode, so that the servopressure and the master pressure are not developed, thus resulting in nofrictional braking force generated in the friction braking devices Bfl,Bfr, Brl, and Brr.

Pressure-Increasing Mode

When the effort on the brake pedal 71 exceeds the frictional brakingforce generating level P2, the hydraulic booster 10 enters thepressure-increasing mode. Specifically, the application of effort to thebrake pedal 71 causes the simulator rubber 34 (i.e., the movable member32) to push the retaining piston 33 to urge the spool piston 23 forward.The spool piston 23 then advances to a front position, as illustrated inFIG. 5 within the movable range against the pressure, as produced by thespool spring 25. Such a front position will also be referred to as apressure-increasing position below.

When the spool piston 23 is in the pressure-increasing position, asillustrated in FIG. 5, the first fluid flow port 23 d is closed by theinner periphery of the spool cylinder 24 to block the communicationbetween the first fluid flow part 23 d and the second spool groove 24 f.This blocks the fluid communication between the servo chamber 10 c andthe reservoir 19.

Further, the spool port 24 c communicates with the third spool groove 23b. The third spool groove 23 b, the first spool groove 24 d, and thefourth spool groove 23 c communicate with each other, so that thepressure in the accumulator 61 (i.e., the accumulator pressure) isdelivered to the servo chamber 10 c through a pressure-increasing flowpath, as defined by the first inner port 12 d, the spool port 24 c, thethird spool groove 23 b, the first spool groove 24 d, the fourth spoolgroove 23 c, the second fluid flow port 23 f, the fluid flow hole 23 e,and the connecting hole 39 d. This results in a rise in servo pressure.

The rise in servo pressure will cause the second master piston 14 tomove forward, thereby moving the first master piston 13 forward throughthe second return spring 18. This results in generation of the masterpressure within the second master chamber 10 b and the first masterchamber 10 a. The master pressure increases with the rise in servopressure. In this embodiment, the diameter of the front and rear seals(i.e., the sealing members 43 and 44) of the second master piston 14 isidentical with that of the front and rear seals (i.e., the sealingmembers 41 and 42) of the first master piston 13, so that the servopressure will be equal to the master pressure, as created in the secondmaster chamber 10 b and the first master chamber 10 a.

The generation of the master pressure in the second master chamber 10 band the first master chamber 10 a will cause the brake fluid to bedelivered from the second master chamber 10 b and the first masterchamber 10 a to the wheel cylinders WCfr, WCfl, WCrr, and WCrl throughthe pipes 51 and 52 and the pressure regulator 53, thereby elevating thepressure in the wheel cylinders WCfr, WCfl, WCrr, and WCrl (i.e., thewheel cylinder pressure) to produce the frictional braking force appliedto the wheels Wfr, Wfl, Wrr, and Wrl.

Pressure-Holding Mode

When the spool piston 23 is in the pressure-increasing position, theaccumulator pressure is applied to the servo chamber 10 c, so that theservo pressure rises. This causes a return pressure that is given by theproduct of the servo pressure and a cross-sectional area of the spoolpiston 23 (i.e., a seal area) to act on the pool piston 23 backward.When the sum of the return pressure and the pressure, as produced by thespool spring 25 and exerted on the spool piston 23, exceeds the inputpressure exerted on the spool piston 23, the spool piston 23 is movedbackward and placed in a pressure-holding position, as illustrated inFIG. 6, that is intermediate between the pressure-reducing position andthe pressure-increasing position.

When the spool piston 23 is in the pressure-holding position, asillustrated in FIG. 6, the spool port 24 c is closed by the outerperiphery of the spool piston 23. The fourth spool groove 23 c is alsoclosed by the inner periphery of the spool cylinder 24. This blocks thecommunication between the spool port 24 c and the second fluid flow port23 f to block the communication between the servo chamber 10 c and theaccumulator 61, so that the accumulator pressure is not applied to theservo chamber 10 c.

Further, the fourth spool groove 23 c is closed by the inner peripheryof the spool cylinder 24, thereby blocking the communication between thefirst fluid flow port 23 d and the second spool groove 24 f to block thecommunication between the servo chamber 10 c and the reservoir 19, sothat the servo chamber 10 c is closed completely. This causes the servopressure, as developed upon a change from the pressure-increasing modeto the pressure-holding mode, to be kept as it is.

When the sum of the return pressure exerted on the spool piston 23 andthe pressure, as produced by the spool spring 25 and exerted on thespool piston 23, is balanced with the input pressure exerted on thespool piston 23, the pressure-holding mode is maintained. When theeffort on the brake pedal 71 drops, so that the input pressure appliedto the spool piston 23 decreases, and the sum of the return pressureapplied to the spool piston 23 and the pressure, as produced by thespool spring 25 and exerted on the spool piston 23, exceeds the inputpressure exerted on the spool piston 23, it will cause the spool piston23 to be moved backward and placed in the pressure-reducing position, asillustrated in FIG. 3. The pressure-reducing mode is then entered, sothat the servo pressure in the servo chamber 10 c drops.

Alternatively, when the spool piston 23 is in the pressure-holdingposition, and the input pressure applied to the spool piston 23 riseswith an increase in braking effort on the brake pedal 71, so that theinput pressure acting on the spool piston 23 exceeds the sum of thereturn pressure exerted on the spool piston 23 and the pressure, asproduced by the spool spring 25 and exerted on the spool piston 23, itwill cause the spool piston 23 to be moved forward, and placed in thepressure-increasing position, as illustrated in FIG. 5. Thepressure-increasing mode is then entered, so that the servo pressure inthe servo chamber 10 c rises.

Usually, the friction between the outer periphery of the spool piston 23and the inner periphery of the spool cylinder 24 results in hysteresisin the movement of the spool piston 23, which disturbs the movement ofthe spool piston 23 in the longitudinal direction thereof, thus leadingto less frequent switching from the pressure-holding mode to either ofthe pressure-reducing mode or the pressure-increasing mode.

Relation Between Regenerative Braking Force and Frictional Braking Force

The relation between the regenerative braking force and the frictionalbraking force will be described below with reference to FIG. 4. When thebraking effort applied to the brake pedal 71 is lower than thefrictional braking force generating level P2, the hydraulic booster 10is kept in the pressure-reducing mode without switching to thepressure-increasing mode, so that the frictional braking force is notdeveloped. The brake system B, as illustrated in FIG. 4, has aregenerative braking force generating level P1 indicative of the brakingeffort applied to the brake pedal 71 which is set lower than thefrictional braking force generating level P2.

The brake system B is equipped with the brake sensor 72. The brakesensor 72 works to measure an amount of stroke of the brake pedal 71.The driver's effort (i.e. the braking effort) applied to the brake pedal71 has a given correlation with the amount of stroke of the brake pedal71. The brake ECU 6, thus, determines whether the braking effort hasexceeded the regenerative braking force generating level P1 or not usingthe output from the brake sensor 72.

When the brake pedal 71 has been depressed, and the brake ECU 6determines that the braking effort on the brake pedal 71 has exceededthe regenerative braking force generating level P1, as indicated in FIG.4, the brake ECU 6, as described above, calculates the targetregenerative braking force as a function of the output from the brakesensor 72 and outputs a signal indicative thereof to the hybrid ECU 9.

The hybrid ECU 9 uses the speed V of the vehicle, the state of charge inthe battery 507, and the target regenerative braking force to computethe actually producible regenerative braking force that is aregenerative braking force the regenerative braking system A is capableof producing actually. The hybrid ECU 9 then controls the operation ofthe regenerative braking system A to create the actually producibleregenerative braking force.

When determining that the actually producible regenerative braking forcedoes not reach the target regenerative braking force, the hybrid ECU 9subtracts the actually producible regenerative force from the targetregenerative braking force to derive an additional frictional brakingforce. The event that the actually producible regenerative braking forcedoes not reach the target regenerative braking force is usuallyencountered when the speed V of the vehicle is lower than a given valueor the battery 507 is charged fully or near fully. The hybrid ECU 9outputs a signal indicative of the additional frictional braking forceto the brake ECU 6.

Upon reception of the signal from the hybrid ECU 9, the brake ECU 6controls the operation of the pressure regulator 53 to control the wheelcylinder pressure to make the friction braking devices Bfl, Bfr, Brl,and Brr create the additional regenerative braking force additionally.Specifically, when it is determined that the actually producibleregenerative braking force is less than the target regenerative brakingforce, the brake ECU 6 actuates the pressure regulator 53 to develop theadditional regenerative braking force in the friction braking devicesBfl, Bfr, Brl, and Brr to compensate for a difference (i.e., shortfall)between the target regenerative braking force and the actuallyproducible regenerative braking force, thereby achieving the targetregenerative braking force.

As described above, when the hybrid ECU 9 has decided that it isimpossible for the regenerative braking system A to produce a requiredregenerative braking force (i.e., the target regenerative brakingforce), the pressure regulator 53 regulates the pressure to be developedin the wheel cylinders WCfl, WCfr, WCrl, and WCrr to produce a degree offrictional braking force through the friction braking devices Bfl, Bfr,Brl, and Brr which is equivalent to a shortfall in the regenerativebraking force.

Operation of Hydraulic Booster in Event of Malfunction of HydraulicPressure Generator

When the hydraulic pressure generator 60 has failed in operation, sothat the accumulator pressure has disappeared, the fail-safe spring 36urges or moves the fail-safe cylinder 12 forward until the flange 12 hof the fail-safe cylinder 12 hits the stopper ring 21 c of the stopper21. The second cylindrical portion 12 c of the fail-safe cylinder 12then blocks the seventh port 11 h of the master cylinder 11 to close thesimulator chamber 10 f liquid-tightly.

When the simulator chamber 10 f is hermetically closed, and the brakepedal 71 is depressed, it will cause the braking effort applied to thebrake pedal 71 to be transmitted from the input piston 15 to theretaining piston 33 through the connecting member 31 and the operatingrod 16, so that the retaining piston 33, the spool piston 23, and thesecond spool spring retainer 39 advance.

Upon hitting of the retaining piston 33 on the stopper 12 m in the failcylinder 12, the braking effort on the brake pedal 71 is transmitted tothe fail-safe cylinder 12 through the stopper 12 m, so that thefail-safe cylinder 12 advances. This causes the pushing member 40 tocontact the retaining portion 14 c of the second master piston 14 or thepressing surface 12 i of the fail-safe cylinder 12 to contact the rearend of the second cylindrical portion 14 b of the second master piston14, so that the braking effort on the brake pedal 71 is inputted to thesecond master piston 14. In this way, the fail-safe cylinder 12 pushesthe second master piston 14.

As apparent from the above discussion, in the event of malfunction ofthe hydraulic pressure generator 60, the braking effort applied to thebrake pedal 71 is transmitted to the second master piston 14, thusdeveloping the master pressure in the second master chamber 10 b and thefirst master chamber 10 a. This produces the frictional braking force inthe friction braking devices Bfl, Bfr, Brl, and Brr to decelerate orstop the vehicle safely.

The depression of the brake pedal 71 in the event of malfunction of thehydraulic pressure generator 60, as described above, results infrontward movement of the fail-safe cylinder 12, thereby causing thefirst spring retainer 29 for the pedal return spring 27 to move forward.This causes the braking effort on the brake pedal 71 not to act on thepedal return spring 27. The braking effort is, therefore, not attenuatedby the compression of the pedal return spring 27, thereby avoiding adrop in the master pressure arising from the attenuation of the brakingeffort.

In the event of malfunction of the hydraulic pressure generator 60, thefail-safe cylinder 12 advances, so that the second cylindrical portion12 c which has the outer diameter c greater than the outer diameter b ofthe first cylindrical portion 12 b passes through the sealing member 45.The master cylinder 11 is designed to have the inner diameter greaterthan the outer diameter c of the second cylindrical portion 12 c forallowing the second cylindrical portion 12 c to move forward.Consequently, when the hydraulic pressure generator 60 is operatingproperly, the outer periphery of the first cylindrical portion 12 b is,as can be seen in FIG. 2, separated from the inner periphery of themaster cylinder 11 through an air gap.

The entire circumferential area of the front end of the sealing member45 is, as clearly illustrated in FIG. 3, in direct contact with thesupport member 59. The inner peripheral surface of the support member 59is in direct contact with the outer peripheral surface of the firstcylindrical portion 12 b of the fail-safe cylinder 12. In other words,the sealing member 45 is firmly held at the front end thereof by thesupport member 59 without any air gap therebetween, thus avoiding damageto the sealing member 45 when the fail-safe cylinder 12 moves forward inthe event of malfunction of the hydraulic pressure generator 60, so thatthe first cylindrical portion 12 b slides on the sealing member 45.

The support member 59 has, as described above, the slit formed therein.The slit makes the support member 59 expand outwardly upon the forwardmovement of the fail-safe cylinder 12, thereby allowing the secondcylindrical portion 12 c to pass through the support member 59. Thesealing member 45 is, as described above, held at the front end thereofby the support member 59, thus avoiding damage to the sealing member 45upon the passing of the second cylindrical portion 12 c through thesupport member 59.

If the accumulator pressure has risen excessively, so that the pressurein the fifth port 11 f has exceeded a specified level, the mechanicalrelief valve 22 will be opened, so that the brake fluid flows from thefifth port 11 f to the sixth port 11 g and to the reservoir 19. Thisavoids damage to the pipe 67 and the hydraulic booster 10.

Mechanism for Achieving Collision Avoidance

The mechanism designed to avoid collision of the system vehicle with anobject will be discussed below. A space enclosed by, as illustrated inFIG. 3, a portion of the outer periphery of the spool piston 23 closerto the top thereof than the in-cylinder portion 23 is, the front surfaceof the retaining piston 33, and the inner periphery of the secondcylindrical portion 12 c of the fail-safe cylinder 12 defines a pilotchamber 12 x. The fluid flow path 12 n and the second inner port 12 eare fluidly connected through a flow path 12 y formed between the outerperiphery of the spool cylinder 24 and the inner periphery of thefail-safe cylinder 12. The pilot chamber 12 x communicates with thesixth port 11 g through the fluid flow path 12 n, the flow path 12 y,and the second inner port 12 e.

The master cylinder 11, as illustrated in FIG. 2, has a hydraulicpressure supply port 11 z leading to the sixth port 11 g. The hydraulicpressure supply port 11 z also connects with the pipe 67 through thepipe 68.

The first solenoid valve 91 is disposed in the pipe 68. The pilotchamber 12 x connects with the accumulator 61 through the first solenoidvalve 91. The first solenoid valve 91 is of a normally-closed type.Specifically, when deenergized, the first solenoid valve 91 keeps thepipe 68 closed. Alternatively, when energized, the first solenoid valve91 opens the pipe 68. The first solenoid valve 91 is controlled inoperation by the brake ECU 6 and works to adjust the accumulatorpressure to a pilot pressure.

The second solenoid valve 92 is disposed in a flow path 95 connectingbetween the pilot chamber 12 x and the reservoir 19. The second solenoidvalve 92 is of a normally-open type and, when deenergized, keeps theflow path 95 opened.

The first solenoid valve 91 and the second solenoid valve 92 are eachimplemented by a linear electromagnetic valve which is capable ofregulating a flow rate of brake fluid passing therethrough or regulatingthe hydraulic pressure of brake fluid flowing upstream thereof.

The first solenoid valve 91, as illustrated in FIG. 8A, consistsessentially of a valve body 91 a, a plunger 91 b, a coil spring 91 c, acoil 91 d, and a core 91 i. The valve body 91 a is made of non-magneticmaterial and has an inlet 91 j and an outlet 91 k leading to the inlet91 j through a fluid path formed therein. The inlet 91 j is formed in anend of the valve body 91 a. The outlet 91 k is formed in a side wall ofthe valve body 91 a. The valve body 91 a has a tapered or conical valveseat 91 e formed on an inner wall thereof. The conical valve seat 91 eis located inside the inlet 91 j. The plunger 91 b consists of aspherical or conical valve body 91 f, an armature 91 g, and a bar 91 hconnecting the valve body 91 f and the armature 91 g. The armature 91 gis formed by a magnetic block made of, for example, an electromagneticstainless steel. The plunger 91 b is disposed inside the valve body 91 ato be slidable to bring the valve body 91 f into hermetic contact withthe valve seat 91 e.

The coil 19 d is installed in the valve body 91 a and located outsidethe periphery of the armature 91 g. The core 91 i is installed away fromthe inlet 91 j within the valve body 91 a and faces the armature 91 g.The core 91 i is made of magnetic material such as electromagneticstainless steel. The coil spring 91 c is installed in the valve body 91a and works as a biasing mechanism to urge the plunger 91 b intoconstant contact with the valve seat 91 e, thereby closing the fluidpath in the valve body 91 a, that is, blocking the fluid communicationbetween the inlet 91 j and the outlet 91 k.

When the coil 91 d is energized, it will produce a magnetic attractionto pull the armature 91 g to the core 91 i, so that the pressurepressing the plunger 91 b against the valve seat 91 e will be decreased,thus causing the fluid path in the valve body 91 a to be opened by thepressure of the brake fluid entering the inlet 91 j. The brake fluid,therefore, stars to flow from the inlet 91 j to the outlet 91 k of thefirst solenoid valve 91. The plunger 91 b is moved to a place where adifference in pressure between the inlet 91 j and the outlet 91 k whichacts on the plunger 91 b is in balance with the pressure which isproduced by the coil 91 d and the coil spring 91 c and exerted on theplunger 91 b (i.e. the sum of the magnetic attraction and the springpressure). The amount of electric current supplied to the coil 91 d isregulated in a duty-factor control mode. Specifically, the pressurewhich presses the valve body 91 f against the valve seat 91 e dependsupon a duty factor of a pulse signal produced by the brake ECU 6,thereby changing the difference in pressure between the inlet 91 j andthe outlet 91 k.

The second solenoid valve 92, as illustrated in FIG. 8B, consistsessentially of a valve body 92 a, a plunger 92 b, a coil spring 92 c, acoil 92 d, and a core 92 i. The valve body 92 a, the plunger 92 b, thecoil spring 92 c, the coil 92 d, and the core 92 i are substantiallyidentical in structure or operation with the valve body 91 a, theplunger 91 b, the coil spring 91 c, the coil 91 d, and the core 91 i ofthe first solenoid valve 91, and explanation thereof in detail will beomitted here.

The coil spring 92 c is, as can be seen in FIG. 8B, disposed so as tourge the plunger 92 b away from the valve seat 92 e. The core 92 i isinstalled close to the inlet 92 j within the valve body 92 a and facesthe armature 92 g. When the coil 92 d is placed in a deenergized state,the coil spring 92 c works to keep the valve body 92 f away from thevalve seat 92 e, thereby opening the fluid path connecting the inlet 92i and the outlet 92 k in the valve body 92 a. When the coil 92 d isenergized, it will produce a magnetic attraction to pull the armature 92g to the core 92 i, so that the plunger 92 b moves toward the inlet 92j, and the valve body 92 f hermetically rides on the valve seat 92 e.This blocks the fluid communication between the inlet 92 j and theoutlet 92 k. As apparent from the above discussion, the second solenoidvale 92 is an electromagnetic relief valve working to control thepressure of brake fluid to be relieved therefrom as a function of anamount of electric current supplied to the coil 92 d.

When a collision avoidance braking mode, as will be described later indetail, is entered, the ECU 6 closes the second solenoid valve 92,thereby hermetically closing the pilot chamber 12 x which leads to thereservoir 19, that is, blocking the fluid communication between thepilot chamber 12 x and the reservoir 19. Subsequently, the ECU 6 opensthe first solenoid valve 91, so that the brake fluid whose pressure isadjusted by the first solenoid valve 91 to the pilot pressure, asclearly illustrated in FIG. 13, flows into the pilot chamber 12 xthrough the hydraulic pressure supply port 11 z, the sixth port 11 g,the second inner port 12 e, the flow path 12 y, and the fluid flow path12 n. The brake fluid, as adjusted to the pilot pressure, then flows tothe pilot-pressure induction path 23 x and to the retaining cavity 33 cof the retaining piston 33. The pilot pressure then acts on the rear endsurface of the in-cylinder portion 23 a of the spool piston 23. Thiscauses a drive force (i.e., a hydraulic pressure), created by theproduct of the pilot pressure and a transverse sectional area (i.e., asealing area) of the spool piston 23, to be exerted on the spool piston23, so that the spool piston 23 moves forward to achieve thepressure-increasing mode, as described already, thereby developing theservo pressure in the servo chamber 10 c.

When a total force that is a combination of a returning force, asderived by multiplying the servo pressure by the transverse sectionalarea (i.e., the sealing area) of the spool piston 23, and the springpressure, as created by the spool spring 25, is balanced with the driveforce, as derived by multiplying the pilot pressure by the transversesectional area of the spool piston 23, the spool piston 23 moves to thepressure-holding position to achieve the pressure-holding mode. When thepilot pressure drops, so that the total force that is a combination ofthe returning force and the spring pressure of the spool spring 25exceeds the above drive force, the spool piston 23 moves backward to thepressure-reducing position, as illustrated in FIG. 3, to establish thepressure-reducing mode, so that the servo pressure in the servo chamber10 c drops. In this way, the pilot pressure and the servo pressure beara proportional relation according to the movement of the spool piston23.

When the pilot pressure is admitted into the pilot chamber 12 x, iturges the pressure-holding piston 33, as clearly illustrated in FIG. 13,backward until it contacts with the C-ring 85. The C-ring 85, asdescribed above, works as a stopper to hold the retaining piston 33 fromsliding backward, thereby eliminating a change in sense of depression ofthe brake pedal 71, as resulting from the backward movement of thepressure-holding piston 33, which the driver of the vehicle feels.

The pressure sensor 99 is disposed in the pipe 68 between the firstsolenoid valve 91 and the hydraulic pressure supply port 11 g and worksto measure the pressure of the brake fluid in the pipe 68, i.e., thepilot pressure inducted into the pilot chamber 12 x and output a signalindicative thereof to the ECU 6.

Collision Avoidance Braking

The collision avoidance braking task to be executed by the brake ECU 6to achieve the collision braking mode will be described below withreference to a flowchart of FIG. 9. The brake ECU 6 works as a collisionavoidance controller When the system vehicle is enabled to run, thebrake ECU 6 enters the collision avoidance braking mode and initiatesthe program of FIG. 9.

After entering the program, the routine proceeds to step S111 whereinthe brake ECU 6 analyzes the outputs from the obstacle detector 97 todetermine whether an obstacle exists frontward in a direction in whichthe system vehicle is heading or not.

If a NO answer is obtained in step S111, the routine repeats theoperation in step S111. Alternatively, if a YES answer is obtained, thenthe routine proceeds to step S112 wherein the brake ECU 6 analyzes theoutputs from the obstacle detector 97 and the wheel speed sensors Sfl,Sfr, Srl, and Srr and determines whether there is a possibility that thesystem vehicle will collide with the obstacle or not. If a NO answer isobtained, then the routine returns back to step S111. Alternatively, ifa YES answer is obtained, then the routine proceeds to step S113.

In step S113, the brake ECU 6 analyzes the outputs from the obstacledetector 97 and the wheel speed sensors Sfl, Sfr, Srl, and Srr todetermine whether the speed V of the system vehicle is lower than orequal to a first reference speed (e.g., 30 km/h) or not and whether thesystem vehicle is running ahead of a collision risk warning location, asillustrated in FIG. 12, or not. If a YES answer is obtained meaning thatthe system vehicle is faster than the first reference speed and existsbetween the obstacle and the collision risk warning location, then theroutine proceeds to step S121. Alternatively, if a NO answer is obtainedmeaning that at least one of the above conditions is not satisfied, thenthe routine proceeds to step S131. The collision risk warning locationis, as illustrated in FIG. 12, defined as where the system vehicle willcollide with an obstacle, as tracked by the obstacle detector 97, unlessthe system vehicle is decelerated at a given rate of, for example, 5m/s² or more when the obstacle is within a radar range (i.e., anobstacle detectable range) of the obstacle detector 97. For instance, ifan obstacle B which has been out of a radar-detectable range in front ofthe system vehicle, as indicated by a broken line in FIG. 12, appears,as represented by an arrow in FIG. 12, within a set distance from thesystem vehicle in the radar-detectable range, the brake ECU 6 determinesthat the system vehicle is closer to the obstacle B than the collisionrisk warning location is, in other words, that the system vehicle is nowbetween the collision risk warning location and the obstacle B. The setdistance is a distance between an obstacle A and the collision riskwarning location at the time when the obstacle detector 97 detects theobstacle A for the first time.

In step S121 the brake ECU 6 selects a second braking map, asillustrated in FIG. 10B, for use in the collision avoidance brakingmode. The second braking map is made so that the level of the pilotpressure will reach a target pilot pressure quickly. Specifically, thesecond braking map sets a rate of increase in the pilot pressure afterthe system vehicle starts to be braked to be greater than that in afirst braking map of FIG. 10A. The target pilot pressure, as set in thesecond braking map, is higher than that in the first braking map of FIG.10A. After step S121, the routine proceeds to step S122 wherein thebrake ECU 6 actuates the warning device 98 to inform the driver of thecollision risk. The routine then proceeds to step S151.

If a NO answer is obtained in step S113, then the routine proceeds tostep S131 wherein it is determined whether the speed V of the systemvehicle is lower than a second reference speed (e.g., 60 km/h) or notusing outputs from the wheel speed sensors Sfl, Sfr, Srl, and Srr. If aYES answer is obtained meaning that the speed V is lower than the secondreference speed, then the routine proceeds to step S132. Alternatively,if a NO answer is obtained meaning that the speed V is higher than orequal to the second reference speed, then the routine proceeds to stepS133. The second reference speed is set greater than the first referencespeed used in step S113.

In step S132, the brake ECU 6 selects the first braking map in FIG. 10A.The first braking map is prepared to have a target pilot pressure lowerthan that in the second braking map. The first braking map is alsoprepared to have a rate at which the pilot pressure is increased afterthe system vehicle starts to be braked to be smaller than in the secondbraking map of FIG. 10B. After step S132, the routine proceeds to stepS142.

In step S133, the brake ECU 6 selects the third braking map in FIG. 10C.The third braking map is made so that the pilot pressure is firstincreased at a lower rate along a line 1, kept constant for a givenperiod of time along a line 2, and then increased at a higher rate alonga line 3 to the target pilot pressure. The rate of increase in the pilotpressure along the line 3 is set greater than that along the line 1.After step S133, the routine proceeds to step S142.

In step S142, the brake ECU 6 analyzes the output from the obstacledetector 97 and determines whether the system vehicle has reached thecollision risk warning location or not. If a NO answer is obtained, theroutine repeats step S142. Alternatively, if a YES answer is obtained,the routine proceeds to step S143.

In step S143, the brake ECU 6 actuates the warning device 98 to informthe driver of the collision risk.

The routine proceeds to step S144 wherein the brake ECU 6 monitors theoutput from the brake sensor 72 and determines whether the driver of thesystem vehicle has depressed the brake pedal 71 within given period oftime (e.g., several seconds) from the initiation of the operation instep S143 or not. If a YES answer is obtained, the routine returns backto step S111. Alternatively, if a NO answer is obtained, then theroutine proceeds to step S145.

In step S145, the brake ECU 6 determines whether the system vehicle hasreached a collision avoidance braking start location, as illustrated inFIG. 11, or not. If a YES answer is obtained, then the routine proceedsto step S151. Alternatively, if a NO answer is obtained meaning that thesystem vehicle has not yet reached the collision avoidance braking startlocation, then the routine returns back to step S143. The collisionavoidance braking start location is a location that is the sum of acollision avoidance interval and the braking distance of the systemvehicle away from the location of the obstacle being now tracked by theobstacle detector 97 toward the system vehicle. The collision avoidanceinterval is an interval between the obstacle and the system vehicle whenthe collision with the obstacle is expected to have been avoided, thatis, the speed of the system vehicle relative to the obstacle is expectedto be decreased to zero. The collision avoidance interval depends upon acurrent speed of the system vehicle relative to the obstacle. Forinstance, when the current relative speed of the system vehicle to theobstacle is lower than 8 km/h, the collision avoidance interval is setto 1 m. When the current relative speed of the system vehicle to theobstacle is higher than or equal to 8 km/h, the collision avoidanceinterval is increased in proportion to the current relative speed of thesystem vehicle. The braking distance is calculated using the currentrelative distance between the system vehicle and the obstacle, therelative speed of the system vehicle to the obstacle, and a selected oneof the first to third braking maps.

In step S151, the brake ECU 6 controls the operations of the firstsolenoid valve 91 and the second solenoid valve 92 so as to bring thepilot pressure, as measured by the pressure sensor 99, into agreementwith the target pilot pressure at a rate, as specified in a selected oneof the first to third braking maps, in the feedback control mode toinitiate collision avoidance braking. Specifically, the brake ECU 6actuates the second solenoid valve 92 in the closing direction and alsoactuates the first solenoid valve 91 in the open direction. The rate ofincrease in the pilot pressure is controlled by the first solenoid valve91. The target pilot pressure (i.e., a target braking force) is createdby the second solenoid valve 92. When the deceleration of the systemvehicle, as derived by the acceleration sensor 96 after the start of thecollision avoidance braking, deviates from that, as calculated in aselected one of the first to third braking maps, the brake ECU 6 worksto control the operations of the first solenoid valve 91 and the secondsolenoid valve 92 to regulate the pilot pressure so as to bring thedeceleration, as measured by the acceleration sensor 96, into agreementwith the calculated deceleration. After step S151, the routine proceedsto step S152.

In step S152, the brake ECU 6 analyzes the output from the obstacledetector 97 to determine whether the risk of collision of the systemvehicle with the obstacle has been eliminated or not. If a YES answer isobtained meaning that the collision with the obstacle has been avoided,then the routine returns back to step S111. Alternatively, if a NOanswer is obtained, the routine repeats step S152.

Frictional Brake Unit in Second Embodiment

FIG. 14 illustrates a frictional brake unit B-2 (i.e., a brake system)of the second embodiment. The same reference numbers as employed in thefirst embodiment will refer to the same parts, and explanation thereofin detail will be omitted here.

The second solenoid valve 92 is installed in the flow path 64 whichconnects the reservoir 19 and the seventh port 11 h. In other words, thesecond solenoid valve 92 is disposed in a flow path extending betweenthe simulator chamber 10 f and the reservoir 19. The first solenoidvalve 91 is disposed in the flow path 69 which connects between aportion of the flow path 64 which is closer to the seventh port 11 hthan the second solenoid valve 92 is and the flow path 67. In otherwords, the first solenoid valve 91 is arranged in a flow path extendingbetween the simulator chamber 10 f and the accumulator 61.

When the collision avoidance braking mode is entered, the ECU 6 closesthe second solenoid valve 92, thereby hermetically closing the simulatorchamber 10 f which leads to the reservoir 19, that is, blocking thefluid communication between the simulator chamber 10 f and the reservoir19. Subsequently, the ECU 6 opens the first solenoid valve 91, so thatthe brake fluid whose pressure is adjusted by the first solenoid valve91 to the pilot pressure flows into the simulator chamber 10 f throughthe flow paths 69 and 64, the seventh port 11 h, and the fourth innerport 12 g. This causes the pilot pressure to act on the pressure-holdingpiston 33, so that the pressure-holding piston 33 and the spool piston23 move forward to achieve the pressure-increasing mode, as describedalready, thereby developing the servo pressure in the servo chamber 10c. The pilot pressure and the servo pressure bear a proportionalrelation.

The pressure sensor 99 is disposed in the flow path 64 between thesecond solenoid valve 92 and the simulator chamber 10 f and works tomeasure the pressure of the brake fluid in the flow path 64, the pilotpressure inducted into the simulator chamber 10 f and output a signalindicative thereof to the ECU 6.

The frictional brake unit B-2 of the second embodiment is, as apparentfrom the above, engineered to perform substantially the same emergencybraking operation as that in FIG. 9.

Beneficial Advantage of Brake System

As apparent from the above discussion, if it is determined that there isa risk of collision with an obstacle present ahead of the system vehicle(i.e., YES in steps S112 in FIG. 9), the brake ECU 6 opens the firstsolenoid valve 91 which is designed to switch between a pressureexerting mode in which the hydraulic pressure of the brake fluid storedin the accumulator 61 (i.e., the accumulator pressure) is exerted on thespool piston 23 as the pilot pressure and a non-pressure exerting modein which the hydraulic pressure of the brake fluid stored in theaccumulator 61 is not exerted on the spool piston 23. When the firstsolenoid valve 91 is opened, so that the pressure-exerting mode isentered, it will cause the spool piston 23 to be moved to a locationwhere the pressure-increasing mode is established, thereby developingthe braking force in the friction braking devices Bfl, Bfr, Brl, andBrr. Basically, the emergency braking is achieved by installing thefirst solenoid valve 91 to selectively exert the accumulator pressure onthe spool piston 23, thus allowing an emergency avoidance braking systemto be constructed with a minimum of equipment and facilitating themountability of the frictional brake unit B or B-2 in automotivevehicles.

Further, if it is determined that there is a risk of collision with anobstacle present ahead of the system vehicle (i.e., YES in step S112 inFIG. 9), the brake ECU 6 closes the normally open type of secondsolenoid valve 92 (i.e., step S151) to hermetically close the pilotchamber 12 x or the simulator chamber 10 f, so that the pilot pressureis developed in the pilot chamber 12 x or the simulator chamber 10 f,thereby ensuring the stability in creating the braking force in thecollision avoidance braking mode.

The first solenoid valve 91 is implemented by a linear electromagneticvalve which is capable of regulating the flow rate of brake fluidfollowing therethrough, thus enabling, as illustrated in FIGS. 10A to10C, a rate of increase in the pilot pressure, that is, a rate ofincrease in braking force applied to the wheels Wfl, Wfr, Wrl, and Wrrto be regulated linearly. The second solenoid valve 92 is alsoimplemented by a linear electromagnetic valve which is capable ofregulating the hydraulic pressure of the brake fluid, thus enabling, asillustrated in FIGS. 10A to 10C, the target pilot pressure, that is, thetarget braking force required to be applied to the wheels Wfl, Wfr, Wrl,and Wrr to be regulated linearly.

If the collision avoidance braking is established by a combination ofthe pressure regulator 53 and the first and second solenoid valves 91and 92 which are both designed by a non-linear electromagnetic valve, anuncontrollable range will be created which is, as illustrated in FIG.15B, a range where it is impossible to control the rate of increase inbraking force and the target braking force. Specifically, the frictionalbrake unit B or B-2 works to open the first solenoid valve 91 fully inthe emergency braking operation, so that the accumulator pressure isinputted directly to the pilot chamber 12 x or the simulator chamber 10f, thus resulting in a sharp rise in the braking force, in other words,the frictional brake unit B or B-2 is incapable of controlling orregulating the rate of increase in the braking force. Additionally, thefrictional brake unit B or B-2 also closes the second solenoid valve 92fully in the emergency braking operation, that is, it is incapable ofcontrolling the target braking force.

Further, when it is required to brake the system vehicle in a slowbraking mode, the pressure regulator 53 is capable of producing thebraking force, but incapable of elevating the braking force rapidlybecause the rate of increase in the braking force depends upon theperformance of the pump or the motor of the pressure regulator 53. Thefrictional brake unit B or B-2 of the above embodiment is, however,equipped with the first and second solenoid valves 91 and 92 implementedby a linear electromagnetic valve and, thus, have the capability ofregulating the rate of increase in braking force and the target brakingforce, as illustrated in FIG. 15A. In other words, the frictional brakeunit B or B-2, therefore, does not have the uncontrollable range shownin FIG. 15B.

The frictional brake unit B of the first embodiment has the pilotchamber 12 x formed in the master cylinder 11. The pilot chamber 12 xserves to apply the pilot pressure to the spool piston 23. The firstsolenoid valve 91 is disposed in the flow path 68 connecting between theaccumulator 61 and the pilot chamber 12 x. This ensures the stability inexerting the pilot pressure on the spool piston 23. The frictional brakeunit B, thus, works to advance the spool piston 23 properly to generatethe servo pressure required to create the braking force in the collisionavoidance braking mode.

The frictional brake unit B-2 of the second embodiment is designed tohave the first solenoid valve 91 disposed in a flow path, as defined byflow paths 69 and 64, which connects between the accumulator 61 and thesimulator chamber 10 f, thereby enabling the pilot pressure to beexerted on the spool piston 23 through the pressure-holding piston 33.The frictional brake unit B-2, thus, works to advance the spool piston23 properly to generate the servo pressure required to create thebraking force in the collision avoidance braking mode.

The frictional brake unit B or B-2 selects one of the first to thirdbraking maps, as illustrated in FIGS. 10A to 10C, depending upon thespeed of the system vehicle and/or whether an obstacle has appearedwithin the set distance in front of the system vehicle, and controls theoperations of the first solenoid valve 91 and the second solenoid valve92 according to the selected one of the first to third braking maps,thus enabling the system vehicle to be decelerated desirably.

Specifically, when it is determined that the speed V of the systemvehicle is lower than or equal to the first reference speed and that anobstacle has appeared the set distance ahead of the system vehicle(i.e., YES in step S113 of FIG. 9), the brake ECU 6 selects the secondbraking map of FIG. 10B and increases the braking force sharply to ahigher level, thereby avoiding the risk of a collision with the obstacleappearing suddenly in front of the system vehicle. Alternatively, whenit is determined that that the speed V of the system vehicle is higherthan the first reference speed, the brake ECU 6 does not select thesecond braking map, thus avoiding instability of behavior of the systemvehicle which typically arises from a sudden increase in the brakingforce.

When the speed V of the system vehicle is greater than or equal to thesecond reference speed (i.e., YES in step S131 of FIG. 9), the brake ECU6 selects the third braking map of FIG. 10C and increases the brakingforce at a lower rate along the line 1 in FIG. 10C at an initial stageof the emergency braking operation in order to ensure the stability ofbehavior of the system vehicle and also minimize the risk of a collisionwith another vehicle trailing the system vehicle. Subsequently, thebrake ECU 6 keeps the braking force at a constant level along the line 2in FIG. 10C and continues to decelerate the system vehicle. Finally, thebrake ECU 6 increases the braking force again at a higher rate along theline 3 in FIG. 10C to eliminate the risk of a collision with theobstacle completely without sacrificing the stability of behavior of thesystem vehicle.

Alternatively, when it is determined that the speed V of the systemvehicle is lower than the second reference speed (i.e., YES in step S131of FIG. 9), the brake ECU 6 selects the first braking map of FIG. 10Aand increases the braking force slowly to a level lower than those inFIGS. 10B and 10C, thereby braking the system vehicle without giving thedriver of the system vehicle a sense of hazard.

The simulator spring 26, as described above, urges the input piston 15backward to function as a brake simulator which applies a reaction forceto the brake pedal 71 to imitate an operation of a typical brake system.The simulator spring 26 is disposed inside the cylindrical cavity 11 pof the master cylinder 11 of the hydraulic booster 10. In other words,the master pistons 13 and 14, the spool valve (i.e., the spool cylinder24 and the spool piston 23), the simulator spring 26, and the inputpiston 15 are arranged in alignment with each other (i.e., in serieswith each other) within the cylindrical cavity 11 p of the mastercylinder 11. This layout facilitates the ease with which the frictionalbrake unit B or B-2 is mounted in the vehicle.

The simulator rubber 34 is disposed away from the retaining piston 33which supports the spool piston 23. This layout keeps the braking effortapplied to the brake pedal 71 from being transmitted to the spool piston23 until the simulator rubber 34 retained by the movable member 32contacts the retaining piston 33. In other words, the frictional brakingforce is not created immediately after the depression of the brake pedal71. After the braking effort exceeds the regenerative braking forcegenerating level P1, as shown in the graph of FIG. 5, the regenerativebraking system A starts developing the regenerative braking force. Thisminimizes the dissipation of thermal energy, into which kinetic energyof the vehicle is converted, from the friction braking devices Bfl, Bfr,Brl, and Brr, thereby enhancing the efficiency in using the kineticenergy of the vehicle as the regenerative braking force through theregenerative braking system A.

The movable member 32 which is disposed behind the spool valve betweenthe retaining piston 33 and the input piston 15 serves as a stopper torestrict the frontward movement of the input piston 15 upon depressionof the brake pedal 71, thereby avoiding damage to the simulator spring26.

The frictional brake unit B and B-2 are engineered so as to switch amongthe pressure-reducing mode, the pressure-increasing mode, and thepressure-holding mode according to the longitudinal location of thespool piston 23, as moved in response to the braking effort on the brakepedal 71, within the spool cylinder 24. In other words, the frictionalbraking force is variably developed by the spool valve that is amechanism made up of the spool piston 23 and the spool cylinder 24. Thisenables the frictional braking force to be changed more linearly thanthe case where the frictional braking force is regulated using asolenoid valve.

Specifically, in the case of use of the solenoid valve, a flow of brakefluid usually develops a physical force to lift a valve away from avalve seat when the solenoid valve is opened. This may lead to anexcessive flow of the brake fluid from the solenoid valve, thusresulting in an error in regulating the pressure of the brake fluid andinstability in changing the frictional braking force. In order toalleviate such a drawback, the brake system B is designed to have thespool piston 23 on which the driver's effort on the brake pedal 71 isexerted and switch among the pressure-reducing mode, thepressure-increasing mode, and the pressure-holding mode as a function ofa change in the driver's effort, thereby developing the frictionalbraking force according to the driver's intention.

The damper 37 is, as illustrated in FIG. 3, installed between theretaining cavity 33 c of the retaining piston 33 and the rear endsurface of the spool piston 23. The damper 37 is deformable orcompressible to attenuate or absorb the impact which results from asudden rise in pressure in the servo chamber 10 c and is transmittedfrom the spool piston 23 to the retaining piston 33, thus reducing theimpact reaching the brake pedal 71 to alleviate the discomfort of thedriver.

Modifications

The braking devices (i.e., the frictional brake units B and B-2) of theabove embodiments are equipped with the brake sensor 72 which measuresthe degree of effort applied to the brake pedal 71 in the form of theamount of stroke of the brake pedal 71, but the brake sensor 72 may bedesigned as a stroke sensor to measure the amount of stroke of the inputpiston 15, the connecting member 31 or the operating rod 16 asrepresenting the degree of effort exerted on the brake pedal 71. Thebrake sensor 72 may alternatively be engineered as a load sensor todetect a degree of physical load acting on the brake pedal 71, the inputpiston 15, the connecting member 31, or the operating rod 16.

The braking devices (i.e., the frictional brake units B and B-2) are, asdescribed above, mounted in the hybrid vehicle equipped with theregenerative braking system A, but the present invention (i.e., thehydraulic booster 10) may be installed in another type of vehicle withno regenerative braking system.

The braking devices (i.e., the frictional brake units B and B-2) use thebrake pedal 71 as a brake actuating member which inputs or transmits thedriver's braking effort to the input piston 15, but may alternativelyemploy a brake lever or a brake handgrip instead of the brake pedal 71.The braking devices may also be used with motorbikes or another type ofvehicles.

While the present invention has been disclosed in terms of the preferredembodiments in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodifications to the shown embodiments which can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

What is claimed is:
 1. A braking apparatus for a vehicle comprising: amaster cylinder having a length with a front and a rear, the mastercylinder including a cylindrical cavity extending in a longitudinaldirection of the master cylinder; an accumulator which connects with thecylindrical cavity of the master cylinder and in which hydraulicpressure of brake fluid is stored; a reservoir which connects with thecylindrical cavity of the master cylinder and in which the brake fluidis stored; a master piston which is disposed in the cylindrical cavityof the master cylinder to be slidable in the longitudinal direction ofthe master cylinder, the master piston having a front oriented towardthe front of the master cylinder and a rear oriented to the rear of themaster cylinder, the master piston defining a master chamber and a servochamber within the cylindrical cavity, the master chamber being formedon a front side of the master piston and storing therein the brake fluidto be delivered to a friction braking device working to apply africtional braking force to a wheel of a vehicle equipped with thisbraking apparatus, the servo chamber being formed on a rear side of themaster piston; a spool valve which is disposed on the rear side of themaster piston within the cylindrical cavity of the master cylinder to beslidable in the longitudinal direction of the master cylinder inresponse to an effort, as produced by a driver of the vehicle, on abrake pedal so as to switch among a pressure-reducing mode, apressure-increasing mode, and a pressure-holding mode, thepressure-reducing mode being to establish fluid communication betweenthe servo chamber and the reservoir, the pressure-increasing mode beingto establish fluid communication between the servo chamber and theaccumulator, the pressure-holding mode being to hermetically close theservo chamber; a first solenoid valve which switches between a pressureexerting mode and a non-pressure exerting mode, the pressure exertingmode being to exert the hydraulic pressure of the brake fluid stored inthe accumulator on the spool valve, the non-pressure exerting mode beingnot to exert the hydraulic pressure of the brake fluid stored in theaccumulator on the spool valve; and a collision avoidance controllerwhich works to determine whether there is a risk of a collision of thevehicle equipped with this braking apparatus with an obstacle or not,when it is determined that there is the risk of the collision, thecollision avoidance controller opening the first solenoid valve to movethe spool valve so as to establish the pressure-increasing mode forcreating the frictional braking force applied to the wheel.
 2. A brakingapparatus as set forth in claim 1, wherein the first solenoid valve isimplemented by a linear electromagnetic valve which is capable ofregulating a flow rate of the brake fluid and works to create a pilotpressure from the hydraulic pressure of the brake fluid stored in theaccumulator, the pilot pressure being exerted on the spool valve.
 3. Abraking apparatus as set forth in claim 1, wherein the master cylinderhas defined therein a pilot chamber which exerts hydraulic pressure ofthe brake fluid on the spool valve, and wherein the first solenoid valveis disposed in a flow path connecting between the accumulator and thepilot chamber.
 4. A braking apparatus as set forth in claim 1, furthercomprising an input piston and a spring, the input piston being disposedbehind the spool valve within the cylindrical cavity of the mastercylinder to be slidable in the longitudinal direction of the mastercylinder, the input piston defining a simulator chamber between itselfand the spool valve within the cylindrical cavity and being subjected tothe effort, as produced by the driver of the vehicle, the spring beingdisposed within the simulator chamber to transmit the effort, asinputted to the input piston, to the spool valve, and wherein the firstsolenoid valve is disposed in a flow path connecting between theaccumulator and the simulator chamber.
 5. A braking apparatus as setforth in claim 3, further comprising a second solenoid valve installedin a flow path between the pilot chamber and the reservoir, and whereinwhen it is determined that there is the risk of the collision, thecollision avoidance controller closes the second solenoid valve.
 6. Abraking apparatus as set forth in claim 4, further comprising a secondsolenoid valve installed in a flow path between the simulator chamberand the reservoir, and wherein when it is determined that there is therisk of the collision, the collision avoidance controller closes thesecond solenoid valve.
 7. A braking apparatus as set forth in claim 5,wherein the second solenoid valve is implemented by a linearelectromagnetic valve which is capable of regulating a flow rate of thebrake fluid.
 8. A braking apparatus as set forth in claim 6, wherein thesecond solenoid valve is implemented by a linear electromagnetic valvewhich is capable of regulating a flow rate of the brake fluid.
 9. Abraking apparatus as set forth in claim 2, wherein the collisionavoidance controller controls an operation of the first solenoid valveaccording to a braking map which represents a relation between a timeelapsed from start of braking and a level of the pilot pressure.
 10. Abraking apparatus as set forth in claim 9, further comprising a speeddeterminer which determines a speed of the vehicle, and wherein thecollision avoidance controller determines the level of the pilotpressure using the braking map since the start of braking as a functionof the speed of the vehicle.
 11. A braking apparatus as set forth inclaim 9, further comprising an obstacle detector working to detectappearance of the obstacle within a set distance in front of thevehicle, and wherein the collision avoidance controller determines thelevel of the pilot pressure using the braking map since the start ofbraking depending upon whether the obstacle has appeared within the setdistance or not.